The present invention relates generally to semiconductor devices and, more particularly, to shielding the leads of semiconductor devices.
Some semiconductor devices have lead frames that include leads, and bond wires that extend between the leads and bonding pads of a die mounted on a paddle or flag of the lead frame. The leads transmit signals, power and ground to/from the die. Two leads that carry signals located near each other may couple to one another. This is referred to as “crosstalk” or “cross-coupling,” where variations in one signal can affect amplitudes of nearby signals.
In order to reduce the coupling between two signals, a ground shield may be provided between the leads. One approach for providing passive, ground shielding is to connect every other lead to ground to avoid coupling. However, this limits the number of leads that may be used to carry signals.
Active shielding is another approach to providing shielding in electronic networks. Active shielding is a shielding technique in which one or more shielding lines transmit signals that are dependent upon the signals transmitted by the guarded line. Capacitive coupling is reduced with active shielding when the shielding lines transmit signals in the same direction as the guarded line and inductive coupling is reduced when the shielding lines transmit signals in the opposite direction as the guarded line. Active shielding is typically implemented using the layouts of one or both of the integrated circuits (dies) or printed circuit boards (PCBs). In low-profile quad flat packages (LQFP), active shielding has not been implemented because conventional LFQP layouts and lead frame arrangements do not allow realization of active shielding. However, without the proper shielding in semiconductor devices, like LQFP devices, unwanted cross-coupling may occur.